Black-fronted Tern and Black-billed Gull colonies, 2019 – 2020 Season

The main points arising from seasonal observations are:

Breeding

• Record number of breeding BBG (1310 nests) – first time since 2000 that 2 colonies located on the river.

• Record number of breeding BFT (120 nests).

• It is likely that frequent flooding of the Waimakariri river (13km to the south) contributed to the increase in BBG numbers, and possibly the same with BFT.

• Despite clearance of weeds from almost 20ha (machine and hand pulling) prior to the season, only one such site (Railway) hosted a gull or tern colony.

• Productivity (number of chicks fledged) of both species was poorer than usual.

• It appears that predation could have been a major cause of low productivity.

  Predation

• In addition to the normal all-year-round berm trapping, a total of 48 additional traps were located close to the two BBG colonies and four main BFT colonies during the season.

• Norway rats appeared to be the most important land-based predator. At the Railway site, >100 BBG chicks were killed and over 20 BFT nests robbed of eggs (and probably chicks). Norway rats were also the dominant trap-catch within the Groyne 9 BFT colony.

• Nine hedgehogs were trapped in a weed-infested area alongside the Groyne 9 colony, but were not caught where there was a water barrier.

• Predation did not appear to be a major issue at the Toppings and Groyne 4 colonies.

• Harrier hawks accounted for the loss of 116 BBG chicks at the Railway colony.

• 20 BBG were found dead under powerlines, which they presumably had hit during flight.

• The remains of 91 BBG chicks, which most likely died from natural causes, were found within

  the Railway colony.

  Conclusions

• Numbers of breeding BBG and BFT were at record levels, but productivity was average (BBG) or poor (BFT).

• Rats and harrier hawks were the main predation dangers, but weed invasion is considered to remain as the biggest long-term threat.

  Recommendations

• Maintain weed control at the most attractive sites, making use of gravel extraction operations where possible.

• Record egg hatch success of BBG and BFT, and Improve techniques for counting chicks.

• Initiate predator control earlier at potential colony sites – particularly targeting rats.

• Install more trail cameras at colonies, and improve design of run-through traps.

• Experiment with harrier hawk deterrents – e.g., strategic use of scarecrows.

• Regularly patrol under powerlines to record bird losses to aerial hits.

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